In modern audio/speech digital signal communication system, digital signal is compressed (encoded) at encoder; the compressed information (bitstream) can be packetized and sent to decoder through a communication channel frame by frame. The system of encoder and decoder together is called CODEC. Speech/audio compression may be used to reduce the number of bits that represent the speech/audio signal thereby reducing the bandwidth (bit rate) needed for transmission. However, speech/audio compression may result in quality degradation of decompressed signal. In general, a higher bit rate results in higher quality, while a lower bit rate causes lower quality.
Audio coding based on filter bank technology is widely used. In signal processing, a filter bank is an array of band-pass filters that separates the input signal into multiple components, each one carrying a single frequency subband of the original signal. The process of decomposition performed by the filter bank is called analysis, and the output of filter bank analysis is referred to as a subband signal with as many subbands as there are filters in the filter bank. The reconstruction process is called filter bank synthesis. In digital signal processing, the term filter bank is also commonly applied to a bank of receivers. The difference is that receivers also down-convert the subbands to a low center frequency that can be re-sampled at a reduced rate. The same result can sometimes be achieved by undersampling the bandpass subbands. The output of filter bank analysis could be in a form of complex coefficients; each complex coefficient contains real element and imaginary element respectively representing cosine term and sine term for each subband of filter bank.
In application of filter banks for signal compression, some frequencies are more important than others. After decomposition, the important frequencies can be coded with a fine resolution. Small differences at these frequencies are significant and a coding scheme that preserves these differences must be used. On the other hand, less important frequencies do not have to be exact. A coarser coding scheme can be used, even though some of the finer details will be lost in the coding. Typical coarser coding scheme is based on a concept of BandWidth Extension (BWE) which is widely used. This technology concept sometimes is also called High Band Extension (HBE), SubBand Replica (SBR) or Spectral Band Replication (SBR). Although the name could be different, they all have the similar meaning of encoding/decoding some frequency sub-bands (usually high bands) with little budget of bit rate (even zero budget of bit rate) or significantly lower bit rate than normal encoding/decoding approach. With SBR technology, the spectral fine structure in high frequency band is copied from low frequency band and some random noise could be added; then, the spectral envelope in high frequency band is shaped by using side information transmitted from encoder to decoder.
In some applications, post-processing at the decoder side is used to improve the perceptual quality of signals coded by low bit rate and SBR coding.